A sintered R-T-B based magnet, including an Nd2Fe14B type compound phase as a main phase, is known as a permanent magnet with the highest performance, and has been used in various types of motors such as a voice coil motor (VCM) for a hard disk drive and a motor for a hybrid car and in numerous types of consumer electronic appliances. As Nd may be partially or entirely replaced with another rare-earth element R and as Fe may be partially replaced with another transition metallic element, the Nd2Fe14B type compound is sometimes represented as an R2T14B type compound. B may also be partially replaced with C (carbon).
As a sintered R-T-B based magnet loses its coercivity at high temperatures, such a magnet will cause an irreversible flux loss when exposed to high temperatures. For that reason, when used in a motor, for example, the magnet should maintain coercivity that is high enough even at elevated temperatures to minimize the irreversible flux loss. To realize that, the coercivity of the magnet at an ordinary temperature needs to be increased or the rate of variation in coercivity to a required temperature needs to be decreased.
It has been known that if Nd that is the light rare-earth element RL in the R2T14B type compound phase is replaced with a heavy rare-earth element RH (which may be Dy and/or Tb), the coercivity will increase. It has been believed to be effective to add such a heavy rare-earth element RH as much as possible to the material alloy of a sintered R-T-B based magnet to achieve high coercivity at high temperatures. However, if the light rare-earth element RL (which may be at least one of Nd and Pr) is replaced with the heavy rare-earth element RH in a sintered R-T-B based magnet, the coercivity certainly increases but the remanence decreases instead. Furthermore, as the heavy rare-earth element RH is one of rare natural resources, its use is preferably cut down.
For these reasons, various methods for increasing the coercivity of a sintered R-T-B based magnet effectively with the addition of as small an amount of the heavy rare-earth element RH as possible have recently been researched and developed in order to avoid decreasing the remanence. The applicant of the present application already disclosed, in Patent Document No. 1, a method for diffusing a heavy rare-earth element RH inside of a sintered magnet body of an R—Fe—B based alloy while supplying the heavy rare-earth element RH onto the surface of the sintered magnet body (which will be referred to herein as an “evaporation diffusion process”). According to Patent Document No. 1, inside of a processing chamber made of a refractory metallic material, the sintered R-T-B based magnet body and an RH bulk body are arranged so as to face each other with a predetermined gap left between them. The processing chamber includes a member for holding multiple sintered R-T-B based magnet bodies and a member for holding the RH bulk body. A method that uses such an apparatus requires a series of process steps of arranging the RH bulk body in the processing chamber, introducing a holding member and a net, putting the sintered magnet bodies on the net, mounting the holding member and the net on the sintered magnet bodies, putting the upper RH bulk body on the net, and sealing the processing chamber hermetically and carrying out an evaporation diffusion.
Patent Document No. 2 discloses that in order to improve the magnetic properties of an Nd—Fe—B based intermetallic compound magnetic material, a powder of Yb metal with a low boiling point and a sintered Nd—Fe—B based magnet compact are sealed and heated in a thermally resistant hermetic container, thereby depositing uniformly a coating of Yb metal on the surface of the sintered magnet compact and diffusing a rare-earth element inside of the sintered magnet from that coating (see, in particular, Example #5 of Patent Document No. 2).